Why Measure Fatigability?
Fatigability is the relative inability of a person to maintain the strength of a muscle contraction over time. In essence, it is a form of weakness, and needs to be considered in any attempt to assess the integrity of a patient's neuromotor system.
Extreme fatigability is usually pretty obvious and is typically reported as a problem by the patient. In some cases, however, fatigability may not be recognized as contributing to poor execution of motor tasks requiring either sustained muscle contraction, or repeated movements at short intervals. If the tongue strength of an individual dealing with this more subtle form of fatigability is tested only a few times, their strength may fall in the normal range. Thus they may initially be able to swallow or speak normally. However, repeatedly swallowing during a normal meal may provoke abnormal swallowing. It is entirely possible that a person with "low normal" tongue strength, as tested with the IOPI, may be recognized as having a significant problem if it is known that this person's tongue is also unusually fatigable.
Thus, the measure of fatigability obtained using the IOPI can provide important clinical information about oral-motor problems.
Using the IOPI to Measure Fatigability
The most thoroughly-tested method to measure tongue fatigue is to determine how long, in seconds, a person can maintain a tongue pressure that is 50% of their maximum tongue pressure. This is done on the IOPI by having the person watch a vertical array of nine lights (LEDs) while they compress the tongue or hand bulb. If they produce 50% of their maximum pressure, then the green (middle) light comes on. If they produce significantly more or less than 50%, then the yellow lights above or below the middle light come on to tell them to adjust their effort to make the green light come on again. The clinician starts the IOPI's timer when the green light first comes on, and then stops it when the person can no longer keep the green light on.
Normal values for tongue and hand fatigue of adult males and females are available in the Manual. In general, normal subjects can hold 50% of their tongue maximum for about 35 seconds and hold 50% of their hand maximum for about 90 seconds. Much remains to be learned about the clinical significance of fatigability as a factor in swallowing and speech problems, but perhaps systematic collection of data on fatigability will reveal currently unrecognized relationships. One aspect of tongue fatigability that is known (Robin et al.) is that it can be greatly modified by specialized tongue use. Accomplished trumpet players, for example, have tongues that are very significantly less fatigable than normals, and the tongues of high school debaters who train themselves to speak intelligibly at very high rates are much less fatigable than their non-debating peers. Interestingly, the tongue strength of people in these specialized groups was completely normal.
An alternative method for measuring fatigability, called the constant-effort task, is to have the person being tested watch the light array only until they see the green light come on and stabilize. They are then told to close their eyes and "keep your effort constant; do not let it get harder or easier to hold this pressure." People are able to do this task quite reliably, and they are comfortable when doing it. The pressure gradually falls over time in a predictable way. The time it takes for the pressure to fall to about one-third of its initial value has been shown to be a valid measure of fatigability (Solomon et al.).
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